A long standing problem in alternating current (AC) inverter design for inverters supplying alternating current to an AC motor is the rise in DC bus voltage when the motor is in an overhauling condition, that is when the motor is generating electric power. The DC bus normally supplies DC power to the inverter. The electric power generated by the motor is delivered by the inverter to the DC bus during periods of motor overhauling. The energy delivered to the DC bus is stored in the bus capacitors, and therefore causes a rise in DC bus voltage. The rise in DC bus voltage can become serious when the rate at which electric power is delivered to the DC bus by the motor exceeds the maximum dissipation rate available to the bus. Under these conditions the DC bus voltage may rise dangerously high and cause damage to electrical components. Particularly, the DC bus voltage may exceed the voltage rating for semiconductor switches or the DC bus capacitors and thereby injure these components.
Methods in the prior art for controlling the DC bus voltage of an AC inverter for driving an AC motor during deceleration of the AC motor include freezing the frequency of the AC current delivered by the inverter to the motor. As the frequency delivered by the inverter decreases the motor decelerates. Regulators of the prior art detect DC bus voltage and freeze the frequency of the inverter when the DC bus voltage reaches a set point value. However, the DC bus voltage frequently overshoots its set point value when the inverter frequency is frozen. The overshoot in DC bus voltage occurs as a result of the slip characteristics of an AC motor. The slip characteristics of an AC motor arise because most AC motors run at a speed slightly less than synchronous speed under motoring conditions. And even though the inverter frequency is frozen at the frequency found when the DC voltage of the bus exceeds a set point value, the motor will continue to decelerate as a result of the slip characteristics to a speed less than the inverter frequency. This continued deceleration of the motor causes an additional increase in DC bus voltage and is therefore responsible for the overshoot of DC bus voltage from the set point voltage.